diff --git a/web/fup/fuphttp/fuphttp.go b/web/fup/fuphttp/fuphttp.go
index b99f28b626..9405e6e995 100644
--- a/web/fup/fuphttp/fuphttp.go
+++ b/web/fup/fuphttp/fuphttp.go
@@ -18,6 +18,7 @@ import (
 	shuncheckedconversions "github.com/google/safehtml/uncheckedconversions"
 	"github.com/gorilla/mux"
 	"gocloud.dev/blob"
+	"hg.lukegb.com/lukegb/depot/web/fup/fuphttp/fngen"
 	"hg.lukegb.com/lukegb/depot/web/fup/hashfs"
 )
 
@@ -40,6 +41,9 @@ type Config struct {
 	// Set one of these, but not both.
 	StorageURL     string
 	StorageBackend *blob.Bucket
+
+	// FilenameGenerator returns a new filename based on the provided prefix and extension.
+	FilenameGenerator fngen.FilenameGenerator
 }
 
 type Application struct {
@@ -49,6 +53,8 @@ type Application struct {
 
 	redirectToBlobstore bool
 	redirectExpiry      time.Duration
+
+	filenameGenerator fngen.FilenameGenerator
 }
 
 func (a *Application) Handler() http.Handler {
@@ -109,10 +115,14 @@ func New(ctx context.Context, cfg *Config) (*Application, error) {
 	a := &Application{
 		redirectToBlobstore: cfg.RedirectToBlobstore,
 		redirectExpiry:      cfg.RedirectExpiry,
+		filenameGenerator:   cfg.FilenameGenerator,
 	}
 	if a.redirectExpiry == 0 {
 		a.redirectExpiry = defaultRedirectExpiry
 	}
+	if a.filenameGenerator == nil {
+		a.filenameGenerator = fngen.PetnameGenerator
+	}
 
 	bkt := cfg.StorageBackend
 	if bkt == nil {